Root filesystem init failure after Linux booting

I met a problem in the end of Linux booting in ARC HS38.Here 's my SW kit information:Linux : 4.4.9 from kernel orgToolchain: gcc version 4.8.4 (ARCv2 ISA Linux uClibc toolchain built on 20160318)

I saw all the return value from Linux while executing /sbin/init (ELF format) is correct.And run the start_thread() in arch/arc/kernel/process.c . But It seems nothing runningwith my /sbin/init.Though I tried with prebuilt rootfs from website and from buildroot. All the results are the same.